I usually buy large bags of Optimum Nutrition's 100% Whey Gold Standard from a popular online bodybuilding site. Can't say the name, but for a cheap price the 153 servings is worth it. But I'm sure you can guess the name of the site through description.

Thinking about buying a giant protein order. Who has done this, where did you use and how was your experience? Who offers the best value?

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Never ordered in bulk but American Whey seems to have decent prices if you can find a code. Still divides down to about $30-$40 per 5lb portion, which can be found here on Nutraplanet, so it would not really save you that much.
I usually just wait til I see a good deal on 5lb tubs here, under $25, and then I back the truck up lol.

Use code "fl3x10" to get a free shirt with your purchase at Mind and Muscle

Subbing for info.
Never ordered in bulk but American Whey seems to have decent prices if you can find a code. Still divides down to about $30-$40 per 5lb portion, which can be found here on Nutraplanet, so it would not really save you that much.
I usually just wait til I see a good deal on 5lb tubs here, under $25, and then I back the truck up lol.